Clutchless Shifting

October 29, 2007 by Rob.

One of the “great debates” for EVers is clutched vs clutchless designs.  The fact that the RAVolt has no clutch was a subject of much interest at the Maker Faire.  The very simple answer to “how does that work?” is that the unpowered motor has very little inertia so the synchronizers in the transmission can handle the work of matching speeds when I shift.  It’s a little more complex in practice.

I choose clutchless primarily because it was a faster and simpler design pattern to implement.  Here are some points:

• positive: less inertia (no flywheel) mean less power loss
• positive: less risk of over revving the motor (no power during shift)
• positive: simpler mating (coupling goes right to clutch disk)
• negative: wears on transmission synchronizers (some transmissions have weak synchros)
• negative: wearing parts are gears, not a replaceable clutch disk.
• negative: shifting takes longer (not suitable for racing)

I’m not really interested in the debate at this point because I put my chips on the table.  Experience will tell me if I made the right decision; however, I’m happy to explain what it’s like to drive clutchless for other EVers.

First, it’s important to realize that EVs don’t shift much.  The RAVolt stays in 2nd from 0 to 35 MPH and does not need to be in neutral while “idling” because the motor is not spinning and wasting our irreplaceable natural resources.  At any speed above 10 MPH, I shift into a cruising gear.  I choose the gear based on the road.  Entering the highway calls for 4th while 3rd works for normal cruising.  I can ride any gear down to 0.  The reason to start in 2nd is because higher gears are not designed to handle to torque of a standing start.  The reason to shift up is to avoid damaging the motor by over revving.

Whew, that’s a lot of wordy preamble.  Here’s the meat: clutchless shifting is just like regular shifting, but a slower.

The “slower” comes from giving the synchronizers time to get up to speed before I mate the actual gears.  The higher the speed difference the slower the shift.  That makes it hard to down shift.  The transmission resists shifting if the speed difference is too great.  In normal shifting from 2nd to 3rd, the motor is slowing down gradually while I’m improving the ratio.  That makes for an easy shift because the motor’s RPM are likely to match the transmission input.   If I’m too slow, chose the wrong gear (2nd for 4th?!), or accidentally rev the motor then the transmission will push back more.

One other observation is that it’s harder to position the shifter for 3rd and 4th.  I think that the transmission’s little bit of push back used to psych me out.  A clutched transmission definitely “drops” into gear with a more solid feeling.

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